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DNA what is it. Pentose sugar (deoxyribose) Phosphate molecule Four nitrogenous bases Pyrimidines: cytosine and thymine Purines: adenine and guanine. Proteins. One or more polypeptides Composed of amino acids

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dna what is it
DNA what is it
  • Pentose sugar (deoxyribose)
  • Phosphate molecule
  • Four nitrogenous bases
    • Pyrimidines: cytosine and thymine
    • Purines: adenine and guanine
  • One or more polypeptides
  • Composed of amino acids
    • 20 amino acids of 64 total known are found in the structure of all plants and animals 6 we can not manufacture from scratch and are essential in diet
    • Directed by sequence of bases along DNA strans 3 consecutive bases = a codon
dna replication
DNA Replication
  • Untwisting and unzipping of the DNA strand
    • Single strand acts as a template for replication and transcription to RNA
  • Complementary base pairing done by action of DNA polymerase
    • Adenine-thymine; cytosine-guanine Chargraf’s rules
  • Any inherited alteration of genetic material
    • Chromosome aberrations major changes in the entire DNA strand and entire piece missing or an extra chromosome or even an extra complete set examples include Cri – du – chat mising piece of #5 or trisomy 21 Downs syndrome extra 21 chromosome
    • Base pair substitution
      • One base pair is substituted for another
      • Silent substitution
        • Substitution that does not result in an amino acid change because genetic code is redundant
        • RNA codons GUU, GUC, GUA, GUG all code for the amino acid valine
  • Frameshift mutation
    • Insertion or deletion of one or more base pairs
    • Causes a change in the entire “reading frame”
    • Examples include sickle cell anemia
  • Spontaneous mutation
    • Mutation that occurs in absence of exposure to known mutagens
  • Mutational hotspots
    • Areas of the chromosomes that have high mutation rates
      • A cytosine base followed by a guanine are known to account for a disproportionately large percentage of disease-causing mutations
  • Agent known to increase the frequency of mutations
    • Radiation
    • Chemicals
  • RNA is synthesized from the DNA template
  • Results in the formation of messenger RNA (mRNA)
  • mRNA moves out of the nucleus and into the cytoplasm
  • Process by which RNA directs the synthesis of a polypeptide
  • Site of protein synthesis is the ribosome
  • tRNA contains a sequence of nucleotides (anticodon) complementary to the triad of nucleotides on the mRNA strand (codon)
  • The ribosome moves along the mRNA sequence to translate the amino acid sequence
  • Somatic cells
    • Contain 46 chromosomes (23 pairs)
    • Diploid cells
  • Gametes
    • Contain 23 chromosomes
    • Haploid cells
      • One member of each chromosome pair
  • Meiosis
    • Formation of haploid cells from diploid cells
  • Autosomes
    • The first 22 of the 23 pairs of chromosomes in males and females
    • The two members are virtually identical and thus said to be homologous
  • Sex chromosomes
    • Remaining pair of chromosomes
    • In females, it is a homologous pair (XX)
    • In males, it is a nonhomologous pair (XY)
  • Ordered display of chromosomes
chromosome aberrations
Chromosome Aberrations
  • Euploid cells
    • Cells that have a multiple of the normal number of chromosomes
    • Haploid and diploid cells are euploid forms
  • When a euploid cell has more than the diploid number, it is called a polyploid cell
    • Triploidy: a zygote having three copies of each chromosome (69)
    • Tetraploidy: four copies of each (92 total)
  • Both triploid and tetraploid fetuses don’t survive
chromosome aberrations1
Chromosome Aberrations
  • Aneuploidy
    • A somatic cell that does not contain a multiple of 23 chromosomes
    • A cell containing three copies of one chromosome is trisomic (trisomy)
    • Monosomy is the presence of only one copy of any chromosome
    • Monosomy is often lethal, but infants can survive with trisomy of certain chromosomes
      • “It is better to have extra than less”
chromosome aberrations2
Chromosome Aberrations
  • Disjunction
    • Normal separation of chromosomes during cell division
  • Nondisjunction
    • Usually the cause of aneuploidy
    • Failure of homologous chromosomes or sister chromatids to separate normally during meiosis or mitosis
autosomal aneuploidy
Autosomal Aneuploidy
  • Partial trisomy
    • Only an extra portion of a chromosome is present in each cell
  • Chromosome mosaics
    • Trisomies occurring only in some cells of the body
autosomal aneuploidy1
Autosomal Aneuploidy
  • Down syndrome
    • Best-known example of aneuploidy
      • Trisomy 21
    • 1:800 live births
    • Mentally retarded, low nasal bridge, epicanthal folds, protruding tongue, poor muscle tone
    • Risk increases with maternal age
sex chromosome aneuploidy
Sex Chromosome Aneuploidy
  • One of the most common is trisomy X. This is a female that has three X chromosomes.
  • Termed “metafemales”
    • Symptoms are variable: sterility, menstrual irregularity, and/or mental retardation
    • Symptoms worsen with each additional X
sex chromosome aneuploidy1
Sex Chromosome Aneuploidy
  • Turner syndrome
    • Females with only one X chromosome
    • Characteristics
      • Absence of ovaries (sterile)
      • Short stature (~ 4'7")
      • Webbing of the neck
      • Edema
      • Underdeveloped breasts; wide nipples
      • High number of aborted fetuses
      • X is usually inherited from mother
sex chromosome aneuploidy2
Sex Chromosome Aneuploidy
  • Klinefelter syndrome
    • Individuals with at least two Xs and one Y chromosome
    • Characteristics
      • Male appearance
      • Develop female-like breasts
      • Small testes
      • Sparse body hair
      • Long limbs
    • Some individuals can be XXXY and XXXXY. The abnormalities will increase with each X.
alterations in chromosome structure
Alterations in Chromosome Structure
  • Chromosome breakage
    • If a chromosome break does occur, physiological mechanisms will usually repair the break, but the breaks often heal in a way that alters the structure of the chromosome
    • Agents of chromosome breakage
      • Ionizing radiation, chemicals, and viruses
alterations in chromosome structure1
Alterations in Chromosome Structure
  • Breakage or loss of DNA
  • Cri du chat syndrome
    • “Cry of the cat”
    • Deletion of short arm of chromosome 5
    • Low birth weight, metal retardation, and microcephaly
alterations in chromosome structure3
Alterations in Chromosome Structure
  • Duplication
    • Presence of a repeated gene or gene sequence
    • Rare occurrence
    • Less serious consequences because better to have more genetic material than less (deletion)
    • Duplication in the same region as cri du chat causes mental retardation but no physical abnormalities
alterations in chromosome structure4
Alterations in Chromosome Structure
  • Inversions
    • Two breaks on a chromosome
    • Reversal of the gene order
    • Usually occurs from a breakage that gets reversed during reattachment
      • ABCDEFG may become ABEDCFG
alterations in chromosome structure5
Alterations in Chromosome Structure
  • Translocations
    • The interchanging of material between nonhomologous chromosomes
    • Translocation occurs when two chromosomes break and the segments are rejoined in an abnormal arrangement
alterations in chromosome structure8
Alterations in Chromosome Structure
  • Fragile sites
    • Fragile sites are areas on chromosomes that develop distinctive breaks or gaps when cells are cultured
    • No apparent relationship to disease
alterations in chromosome structure9
Alterations in Chromosome Structure
  • Fragile X syndrome
    • Site on the long arm of the X chromosome
    • Associated with mental retardation; second in occurrence to Down syndrome
    • Higher incidence in males because they have only one X chromosome
  • Gregor Mendel
    • Austrian monk
    • Garden pea experiments
    • Mendelian traits
  • Locus
    • Position of a gene along a chromosome
  • Allele
    • A different form of a particular gene at a given locus
    • Example: Hgb A vs. Hgb S
    • Polymorphism
      • Locus that has two or more alleles that occur with appreciable frequency
  • Homozygous
    • Loci on a pair of chromosomes have identical genes
    • Example
      • O blood type (OO)
  • Heterozygous
    • Loci on a pair of chromosomes have different genes
    • Example
      • AB blood type (A and B genes on pair of loci)
  • Genotype (“what they have”)
    • The genetic makeup of an organism
  • Phenotype (“what they demonstrate”)
    • The observable, detectable, or outward appearance of the genetics of an organism
  • Example
    • A person with the A blood type could be AA or AO. A is the phenotype; AA or AO would be the genotype.
  • If two alleles are found together, the allele that is observable is dominant, and the one whose effects are hidden is recessive
  • In genetics, the dominant allele is represented by a capital letter, and the recessive by a lowercase letter
  • Alleles can be co-dominant
  • Carrier
    • A carrier is one that has a disease gene but is phenotypically normal
    • For a person to demonstrate a recessive disease, the pair of recessive genes must be inherited
    • Example
      • Ss = sickle cell anemia carrier
      • ss = demonstrates sickle cell disease
  • Used to study specific genetic disorders within families
  • Begins with the proband
single gene disorders
Single-Gene Disorders
  • Recurrence risk
    • The probability that parents of a child with a genetic disease will have yet another child with the same disease
    • Recurrence risk of an autosomal dominant trait
      • When one parent is affected by an autosomal dominant disease and the other is normal, the occurrence and recurrence risks for each child are one half
single gene disorders1
Single-Gene Disorders
  • Autosomal dominant disorder
    • Abnormal allele is dominant, normal allele is recessive, and the genes exist on a pair of autosomes
single gene disorders2
Single-Gene Disorders
  • Autosomal dominant traits
single gene disorders3
Single-Gene Disorders
  • Autosomal dominant trait pedigree
  • The percentage of individuals with a specific genotype who also express the expected phenotype
    • Incomplete penetrance
      • Individual who has the gene for a disease but does not express the disease
      • Retinoblastoma (eye tumor in children) demonstrates incomplete penetrance (90%)
  • Expressivity is the variation in a phenotype associated with a particular genotype
  • This can be caused by modifier genes
  • Examples:
    • von Recklinghausen disease
    • Autosomal dominant
    • Long arm of chromosome #17
    • Disease varies from dark spots on the skin to malignant neurofibromas, scoliosis, gliomas, neuromas, etc.
single gene disorders4
Single-Gene Disorders
  • Autosomal recessive disorder
    • Abnormal allele is recessive and a person must be homozygous for the abnormal trait to express the disease
    • The trait usually appears in the children, not the parents, and it affects the genders equally because it is present on a pair of autosomes
single gene disorders5
Single-Gene Disorders
  • Autosomal recessive disorder recurrence risk
    • Recurrence risk of an autosomal dominant trait
      • When two parents are carriers of an autosomal recessive disease, the occurrence and recurrence risks for each child are 25%
  • Mating of two related individuals
  • Dramatically increases the recurrence risk of recessive disorders
sex linked disorders
Sex-Linked Disorders
  • The Y chromosome contains only a few dozen genes, so most sex-linked traits are located on the X chromosome and are said to be X-linked
sex linked disorders1
Sex-Linked Disorders
  • Sex-linked (X-linked) disorders are usually expressed by males because females have another X chromosome to mask the abnormal gene
  • X-linked recessive
    • Most X-linked disorders are recessive
    • Affected males cannot transmit the genes to sons, but they can to all daughters
    • Sons of female carriers have a 50% risk of being affected